In the ingredient descriptions: Good means that I like to see this in a product's list of ingredients. Okay means this product appears safe for a curly person like me to use. Caution means that this ingredient may not be good in some hair care products, or for some people. Avoid means this ingredient may hurt your hair. If you see this ingredient in a hair product, it's best to put it down and walk away.

Glycerin(aka Glycerine, Glycerol)Good
Glycerin is used as a humectant, and also to add slip in a conditioner to make the product easier for combing. It's also put in products to keep the products from drying up in the container.

It's known to be non toxic, and doesn't cause skin irritation (Winter says it may be irritating to the mucas membranes in concentrated solutions, but when used normally, it's non-irritating and non-allergenic.). (Fun fact: It's actually a byproduct of regular human metabolism).

Glycerin is a clear, thick liquid which is completely water soluble. It occurs in nature in animal fats, and is often used to make glycerin soap (when a highly alkaline substance is added to it). It can also be made synthetically [Hunting (Shampoo) pg 238-239].

Glycerin is a by-product of soap making. It's a sweet-tasting, oily liquid (but I don't recommend drinking it, or course), made from adding something basic to a fat or an oil. It's a humectant because it absorbs water from the air, and it can help products spread better[Winter (7th ed) pg 256].
—This is great for combing, but I’ve found that once it dries, it’s gone, so it shouldn’t be too close to the top of the ingredient list (after water). You want it for slip, but you need other ingredients to help keep your hair calm and together after this dries—TSource(s): HuntingWinter